It's not possible to send an email to just a few specific contacts on a list. You'll have to do is generate a brand new list that has only the contacts you want to mail to, and then mail to it.
The easiest way to do that would be to download the full list to an Excel spreadsheet, remove the contacts that you don't want to be included, and then upload a new list with just the contacts that you want to mail.
For instructions on how to download a list from your account, see here:
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